Conjugative plasmids that mediate resistance to antibiotics (R plasmids) among Streptococci generally fall into one of two categories. The large (50-80 kilobases [kb]), narrow host-range, R plasmids carry three or more resistance genes and transfer in broth culture, but only to members of a unique serological group. The small (22-30 kb) broad host-range, R plasmids code for only one or two resistance traits and can be transferred on membrane filters to virtually all species of Streptococcus. A restriction endonuclease map of the 80.7 kb narrow host-range Streptococcus faecalis plasmid, pJH1, was constructed and the locations on the map of the genes for resistance to kamamycin, streptomycin, tetracycline and erythromycin (Em) were determined. Similar plasmids isolated from S. faecalis strains of human and animal origin were found to share greater than 90% (human) and 50% (animal) DNA sequence homology with pJH1. The Em resistance gene of pJH1 and that of three human and three animal streptococcal isolates were located on the same transposable element, Tn917. The locations of the Em resistance gene, transfer region and replication functions were established on a previously constructed map of the 26.5 kb broad host-range plasmid, pAMBeta1. The results of cloning experiments permitted the localization of the replication origin and copy control region of pAMBeta within a 2.95 kb segment of DNA. A recent extension of our research program to the anaerobic genera, Bacteroides and Bifidobacteria, has revealed the presence of R plasmids coding for the same resistance phenotypes as those mediated by pJH1 and pAMBeta1. Cloned resistance genes from these two plasmids are being used for comparative studies with their counterparts on plasmids isolated from the anaerobic strains.